1. Field of the Invention
The present invention relates to reduction of cogging torque of a brushless DC motor and particularly provides a brushless DC motor capable of readily reducing the cogging torque without sacrificing the electrical characteristics.
2. Description of Related Art
In a conventional brushless DC motor (hereinafter referred to as the xe2x80x9cmotorxe2x80x9d), cogging torque is generated due to the presence of slots provided for windings. More specifically, field magnetic flux that is generated from a magnetic pole of a rotor during relative movement of the rotor and a stator changes periodically whenever a magnetic pole of the rotor crosses a slot opening of the stator, and changes the magnetic flux distribution in gaps. Therefore, the cycle and magnitude of this cogging torque depend on the number of slots formed in the stator and the number of magnetic poles of the rotor, and the waveform thereof with respect to the rotation angle changes largely depending on the slot openings and the shapes and dimensions of the magnetic poles of the rotor.
Conventionally, various methods have been proposed for the reduction of cogging torque, and a generally used method smoothes the change in the magnetic flux interlinking arbitrary stator teeth (hereinafter referred to as xe2x80x9cteethxe2x80x9d) by making magnetic spatial gaps between the rotor and the stator larger and unequal at both ends of the rotor""s magnetic pole in the rotating direction of the rotor. Moreover, by skewing the magnetic pole of the rotor in the direction of the rotation axis, the change in the interlinkage magnetic flux interlinking the stator that is caused when the interpole portion of the rotor crosses the teeth is reduced.
The reduction of cogging torque by means of unequal gaps is usually implemented by shaping the permanent magnets, and the shape needs to be changed according to the tooth shape and the slot opening size. Furthermore, in a type of motor incorporating permanent magnets in the rotor, attempts to reduce cogging torque were made by changing the curvature in the shape of the outer rim of the rotor, but, under the present conditions, a large number of trial manufacture and various analysis are carried out to determine the shape of the permanent magnets and the curvature of the outer rim of the rotor. Although the cogging torque was considerably improved by such attempts, it was hard to say that the improvement was sufficient. The more the reduction of the cogging torque by such measures, the wider the gap in the interpole portion of the rotor, resulting in a lowering of interlinkage magnetic flux from the rotor to the stator.
In addition, there is another method in which the magnetic pole of the rotor is skewed as mentioned above. In this case, an advantageous effect can be expected if the angle of skew is large, but the effective magnetic flux of the magnetic pole is reduced in proportion to the skew angle and impairs the motor characteristics. From the view point of the electrical characteristics as the motor, the permanent magnet in a portion where skew is present does not function as effective magnetic flux in the motor performance, and it can be said that a useless permanent magnet is used.
Further, in recent years, many motors use rare-earth permanent magnets of high magnetic flux density for the purposes of achieving small-sized, high-performance motors, and the permanent magnets have also become smaller in size. Therefore, the magnetic loading increases and consequently cogging toque itself becomes larger, and it has become difficult to take sufficient countermeasures by the cogging torque reducing methods of conventional structures. Additionally, in the method for shaping permanent magnet, it is necessary to shape small-sized permanent magnets with high precision, and it is difficult to shape such permanent magnets. In the method adopting skewing, if skewing is to be implemented by a combination of permanent magnets, a permanent magnet in a segment is itself skewed, and thus this method is completely unsuitable for mass-production. FIG. 1 shows a perspective view of a rotor implemented by a combination of skewed permanent magnets. In FIG. 1, a rotor is constructed by arranging four permanent magnets having a skew angle xcex8S in the direction of the rotation axis on the surface of a rotor in the circumferential direction.
Alternatively, in a method adopting no skew in the shape of a permanent magnet, a ring-shaped permanent magnet is sometimes used. In this method, the ring-shaped permanent magnet is electrically skewed by polarization with a skewed magnetic yoke. However, as described above, since a useless area, that is, a skewed portion, is present, the problem of using a magnet that is not concerned in the motor characteristics is not solved.
The present invention was made with the aim of solving the above problems, and an object of the invention is to provide a brushless DC motor capable of readily reducing cogging torque without sacrificing the electrical characteristics.
A brushless DC motor according to the present invention is a brushless DC motor comprising a rotor to which a permanent magnet is attached; and a stator having a plurality of slots, and is characterized in that, when an effective opening angle of magnetic pole of the rotor that was set to a predetermined opening angle whose base is a center of a shaft hole of the rotor (hereinafter all opening angles used in the present invention refer to opening angles whose base is the center of the shaft hole of the rotor) is xcex82 and a salient area opening angle of magnetic pole formed within the effective opening angle xcex82 of magnetic pole is xcex83, the opening angles xcex82 and xcex83 are set so that cogging torque generated by the effective opening angle xcex82 of magnetic pole and cogging torque generated by the salient area opening angle xcex83 of magnetic pole are mutually in antiphase.
Moreover, a brushless DC motor according to the present invention is characterized in that the effective opening angle xcex82 of magnetic pole of the rotor is not smaller than an electrical angle of 120xc2x0 whose base is a pole center of the effective opening angle xcex82 of magnetic pole of the rotor and is an opening angle formed with both edges of stator teeth closest to the electrical angle of 120xc2x0; likewise the salient area opening angle xcex83 of magnetic pole of the rotor is not larger than the electrical angle of 120xc2x0 whose base is the pole center of the effective opening angle xcex82 of magnetic pole of the rotor and is an opening angle formed with both edges of stator teeth closest to the electrical angle of 120xc2x0; and a recessed area wider than a gap in the salient area opening angle xcex83 of magnetic pole of the rotor is formed at least in a portion of the effective opening angle xcex82 of magnetic pole beyond the salient area opening angle xcex83 of magnetic pole.
Furthermore, a brushless DC motor according to the present invention is characterized by having a relationship
0.75xe2x89xa6{(g1/g2)2+1}/2 less than 1xe2x80x83xe2x80x83(1)
where g2 is a gap between a recessed area of the effective opening angle of magnetic pole xcex82 of the rotor and the stator, and g1 is a gap between the rotor and the stator in any area of the salient area opening angle xcex83 of magnetic pole.
In addition, a brushless DC motor according to the present invention is characterized in that the salient area opening angle xcex83 of magnetic pole of the rotor is formed by a magnetic material holding a permanent magnet therein.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.